This invention relates in general to clutches and, more specifically, to friction for use in driving propellers and the like.
Rotating propellers, such as are used with boats, air-moving fans, model aircraft and the like, are generally driven by a motor through a drive shaft. Sometimes the propeller strikes an object or becomes entangled in material such as seaweed, suddenly and severely increasing resistance to rotation. Where the drive shaft is rigidly fastened to the propeller, severe damage to the propeller, drive shaft, bearings, motor, etc., may result.
In order to prevent or alleviate such damage, a variety of devices, such as clutches and shear pins, have been used between drive shafts and propellers to disengage them in the event of a sudden increase in resistance to propeller rotation.
Shear pins and the like usually include a rigid fastener between drive shaft and propeller. The fracture resistance of the fastener is selected to be sufficiently low so as to break before damage to other drive components occurs. While generally effective in preventing damage, these devices totally disable the propeller until repairs, including replacement of the fastener, can be made. Often the repairs are costly, time-consuming and require special equipment unavailable in the field.
A variety of clutch assemblies have been designed to allow direct drive under normal conditions and relative slippage between drive shaft and propeller under high rotation resistance conditions. Many such clutches utilize a tube of rubber-like material bonded or vulcanized around an internally splined drive shaft. This assembly is forced into a propeller bore which has an internal diameter slightly less than the free outside diameter of the rubber tube, so as to assure sufficient pressure between tube and bore to provide drive friction. While such clutches are often effective, any delamination of rubber tube and splined tube will permit excessive propeller slippage and require disassembly of the drive assembly and replacement of the rubber tube/splined tube combination. Also, the diameter of the rubber tube can be only slightly larger than the bore to permit installation without damaging the inter-tube bond. Further, when the rubber tube exterior becomes worn, the entire assembly of splined tube and rubber tube must be replaced.
Instead of bonding the splined tube to the rubber tube, clutches has been designed with longitudinal ridges or keys extending out from the splined tube into corresponding recesses in the rubber tube to lock the tubes together during rotation. Typical of such clutches is that described in U.S. Pat. No. 2,962,312. These clutches, however, leave the rubber tube free to slide longitudinally along the splined tube in at least one direction, making installation and/or removal difficult where the rubber tube is oversize to improve friction characteristics.
With high motor power, clutches must be able to resist slipping during acceleration and other moderately varying rotational forces while promptly disengaging when a predetermined force level is exceeded. Prior rubber tube clutches tend to slip excessively, since the maximum friction between bore and rubber tube is limited by limits on compression of the rubber tube during installation.
Thus, there is a continuing need for improved propeller drive clutches of improved simplicity, reliability and effectiveness.